1. Field of the Invention
In general, the present invention relates to welding techniques, and more particularly to techniques for controlling the force on a resistance welding machine.
2. Statement of Related Art
Resistance welding machines are well known in the art. In a resistance welding process, two pieces of material are held by clamps so they are touching or are nearly touching. An electric current is then delivered through these pieces. The current heats up the interface between the two pieces causing the material to become soft. At that point, the machine rapidly brings the two pieces together which forges the weld. Flash welds and butt welds are two common types of resistance welding techniques.
In certain welding applications, such as railroad rails, it is desirable to have welds forged to a specific force. By forging to a specific force instead of a distance, each weld produced will forge to the correct distance according to the amount of heat that is in the part. Since the actual amount of heat introduced into the part tends to vary, this forging technique leads to more constant welds.
In other welding applications, such as flash welding parts with large cross sections, it is desirable to preheat the parts prior to flashing in order to minimize the amount of material consumed in the flash and to decrease welding time. Preheating is generally accomplished by forcing the two pieces together for some time and then separating them. When the pieces are in contact, the electrical current will cause the pieces to heat according to their resistance. By holding the two pieces together at a constant force, the resistance between the parts can be made more consistent, which allows for the heat produced in the pieces to be controlled more precisely.
In other welding applications, such as butt welding, it is desirable to control the force with which the parts are held together. The butt weld requires that the two parts be in solid contact with each other. When current is delivered through the parts, the interface between the parts heats up due to its electrical resistance. It is critical to the welding process that this resistance not be excessive so as to cause overheating of the parts. If the parts overheat, there is a risk ejecting material from the joint in the form of sparks. These sparks are known as flashing. Flashing unevenly removes heat and material from the joint, lowering its quality, or even preventing proper forging. In most cases, the parts will have to be scrapped. To avoid these undesirable results, butt welding equipment must incorporate a method for controlling the force between the pieces. Further this force may be modified during the course of the weld depending on the requirements of the weld. It is also important that the force be achieved quickly with minimal overshoot to prevent flashing.
In all these welding applications, the welding machine typically utilizes a hydraulic servo valve that controls the movement of one of the parts being welded. By varying the flow of oil, the servo valve also applies a force to the movable part. By maintaining a constant force as applied by the servo valve, the two pieces are maintained in contact with each other thereby reducing the risk of overheating of the parts at the interface.
Resistance welding machinery incorporate various techniques to control the force between the pieces. The simplest technique is to use compressed air instead of hydraulic oil. This technique has limitations in that it is only suitable for small welding applications due to the relatively low pressure of compressed air. Also this technique is generally stable only at a specific pressure which makes it difficult to quickly change pressure to a changing force setpoint. Another technique uses a standard Proportional Integral Derivative (PID) -type controller to control the servo valve. This technique has limitations in that it responds quickly to a change in force but overshoots the desired target in an oscillatory manner. This characteristic overshooting can be minimized at the expense of the quickness of response. Other limitations to this technique are that it only uses knowledge of the overall force and a history of previous force. This would be sufficient if there were a constant force to be acted upon. In resistance welding machinery, the force is being applied to an object whose characteristics are being changed over time due to heating. Since the PID control has no knowledge on the changes in the parts, and is tuned for a specific situation, it will not behave optimally in all situations.
It is therefore an object of the present invention to provide an improved welding technique for controlling the force supplied by the servo valve.